The present invention relates to a refrigerating machine such as a refrigerator or an air-conditioner, using a vapor compression refrigerating cycle.
These years, due to a tendency of increase demands of prevention of earth warming and reduction in running cost, energy saving as to refrigerators and air-conditioners which causes energy consumption in a large part in tenants and buildings has been promoted. In order to enhance the efficiency of a refrigerating machine such as an air-conditioner or a refrigerator and to lower the costs thereof, as disclosed in JP-A-2001-227778, it has been known to use a built-in magnet synchronous motor for driving a compressor, having an armature incorporating a squirrel cage conductor and a permanent magnet so that the motor serves not only as an induction motor but also a synchronous motor.
In the above-mentioned conventional technology, of a plurality of compressors used in a refrigerating machine, the one compressor which is a variable speed compressor driven by a built-in magnet synchronous motor, is adapted to be subjected to an inverter drive, but the other one or more compressors which are constant speed compressors are driven by a commercial power source. However, although the compressor subjected to an inverter drive is driven with a relatively high degree of efficiency in the above-mentioned configuration, it has been not yet known in detail how to enhance the overall efficiency of the refrigerating machine.
In the case of synchronous operation of the built-in magnet synchronous motor, overload operation causes the built-in magnet synchronous motor to produce an insufficient torque which deteriorates the synchronous operation so that the armature stalls, result in running of abnormal current therethrough, and accordingly, there would cause a risk of occurrence of a dielectric breakdown of the armature windings.